In many manufacturing processes, individual sheet metal parts are fabricated with locating holes. Specifically, locating pins extend through these holes to hold the sheet metal parts in position relative to each other and to the overall assembly during the welding process. Thus, accurate positioning of the locating pins is necessary to assure consistent assembly.
In some situations, stationary locating pins may be fixed to the frame of the relevant manufacturing equipment. However, in many manufacturing situations, the locating pins must be retracted from the completed sheet metal assembly so that it can progress to the next station. In these latter situations, the locating pin(s) are commonly mounted on a linear actuator.
A linear actuator typically comprises a housing which defines an internal bore and a piston-rod assembly which moves within the internal bore in response to fluid pressure. One end of the rod is attached to the piston. The other leading end of the rod (which extends beyond the housing) includes pilot holes, flats and/or threaded passages for securing a locating pin thereto.
During the locating and/or welding process, it is important that the piston-rod assembly not rotate relative to the housing. This non-rotation is crucial to insuring that the working position of the locating pin is reliable and repeatable. Rotational issues are magnified when it is necessary for a locating pin to be attached to an actuator with an offset in order to allow the pin to fit around some other part of the equipment during sheet metal working operations.
One technique traditionally used to prevent rotation of a piston-rod assembly in a linear actuator is provide the rod and corresponding bearing surface (of the internal bore) with a rotation-preventing cross-sectional geometry.
For example, the rod/bearing surface can be fabricated having a square cross-sectional geometry. However, such polygonal arrangements are difficult to fabricate in that consistently matching a square bearing to a square shaft in a high production environment is technically challenging. Additionally, even if fabrication issues are ignored, such polygonal arrangements tend to present wear problems. Specifically, whenever torque is applied to the shaft (as from an offset locating pin) the four corners of the shaft will continuously contact the bearing surface thereby making these minimal areas of the rod extremely susceptible to wear. Significantly, replacement of the worn parts usually requires disassembly of the housing components, disassembly of the piston-rod assembly, and replacement of the entire rod.
Another rotation-preventing cross-sectional geometry which is commonly used is a round shaft with a circumference-interrupting flat and a circular bearing surface with a corresponding flat. This shaft design is much easier to manufacture than the square shaft because the flat-grinding process is controllable, even in a high production environment. However, these shafts also tend to wear rapidly because there is only a single point of contact between the flat and the bearing when a torque is applied to the shaft. Again, significantly, replacement of the worn parts usually requires disassembly of the housing components, disassembly of the piston-rod assembly, and replacement of the entire rod.
Instead of the rod, the piston of the piston-rod assembly, and the corresponding piston chamber, may be made with a rotation-preventing shape. For example, the piston having an oval, rather than circular cross-sectional shape may be used thereby geometrically preventing rotation of the piston within its chamber. Again, significantly, replacement of the worn parts usually requires disassembly of the housing components, disassembly of the piston-rod assembly, and replacement of the entire piston.
Accordingly, the inventors appreciated that a need remains for a rotation-preventing device which does not require non-circular piston-rod components, which does not increase the axial length of the actuator, and/or which has bearing surfaces which may be easily accessed, inspected repaired, and/or replaced.